The present invention relates to a method of sensing the rate at which a material is converted into the plasma state in an ion plating process and deposited upon a member which is to be plated with a thin film, whereby the final thickness of a plated thin film can be controlled either by varying the duration of the plating operation in accordance with the measurement results or by continuously controlling the rate of deposition to a predetermined value in accordance with continuous monitoring of a measured value.
Ion plating is a widely utilized process for forming a thin film of evaporatively deposited material which is attached with a high degree of strength to a body upon which the plating is performed. For brevity of description, such a body will be referred to in the following as a substrate. The ion plating process has the further advantage of rapidity, so that a high degree of productivity can be attained. In ion plating, an evaporation source is positioned in proximity to a material which is to be evaporatively deposited (this material being referred to in the following as the plating material, for brevity of description), together with a probe and the substrate, the evaporation source, substrate and probe being positioned spaced apart from one another within an evacuated chamber containing a gas which is at a very low pressure. The evaporation source applies heat to the plating material, causing the plating material to evaporate, and a high positive DC voltage or high AC voltage is applied to the probe to cause ionization of the plating material. The electrons produced by this ionization collide at high speed with molecules of the plating material within the chamber, thereby converting the plating material into a plasma. The substrate is held at a potential such that the plating material, in plasma form, is attracted to and deposited thereon, to thereby form a thin film on the substrate.
Such a method has the disadvantage in the prior art that it is difficult to measure the rate at which the thin film is being formed while ion plating is in progress, and hence to set the final thickness of the thin film to a desired value, since the amount of plating material which is actually deposited upon the substrate within a specific time interval (referred to in the following as the effective evaporation amount) will vary in response to variations in certain operating conditions such as the pressure of the gaseous atmosphere within the evacuated chamber. A method which has been adopted to some extent is based upon disposing a quartz crystal vibrator of a quartz crystal oscillator circuit within the evacuated chamber, and monitoring changes which take place in the frequency of oscillation of the oscillator circuit due to plating material being deposited upon the quartz crystal vibrator. However with certain types of plating material such as titanium, such a method has displayed severe disadvantages as will be discussed in more detail hereinafter. There is therefore an urgent requirement for a simple and effective method of measuring the effective evaporation amount of the plating material in an ion plating process, to enable control of the final thickness of a thin film produced by such a process to a predetermined value.